56 VERTEBRATE RESPIRATION 



Upon the CO 2 content of the blood leaving the lung which is the 

 same as that in the systemic circulation. Zeuthen has calculated 

 that during inspiration 29-48 per cent of the air reaches the air 

 sacs by way of the parabronchi. Of the expired air, 38 to 67 per 

 cent passes by the same route. 



The existence of a ww/directional flow through the para- 

 bronchi in an anterior direction was suggested by the aggrega- 

 tion of charcoal at the origin of the posterior secondary bronchi 

 when it was injected into either the trachea or the abdominal 

 air sacs. This observation, and experiments on glass models 

 together with experiments on the movement of fluids in the 

 fixed lung produced by changes in pressure of the tracheal 

 cavity, are the main lines of evidence which support this theory. 

 The suggested mechanism emphasises the importance of a 

 deflector plate on the mesobronchus opposite the point of origin 

 of the posterior group of secondary bronchi. It also supposes 

 that during inspiration air is drawn out of the anterior secondary 

 bronchi by an aerodynamic eff'ect resulting from the rapid flow 

 of air down the mesobronchus, rather like the action of a filter- 

 pump. 



As yet the evidence for both theories is inconclusive and until 

 observations based upon X-ray cinematography of the lung are 

 carried out there seems little hope of any final resolution of this 

 problem. Nevertheless, it is clear that the mechanism of ventila- 

 tion of the lungs by means of a bellows-like action of the air sacs 

 is a very adaptive one and efficient relative to those which utilise 

 a tidal mechanism without the possibility of a continuous 

 renewal of the gas at the respiratory surface. 



(d) MAMMALS 



The body cavity of mammals is divided into perivisceral, peri- 

 cardial, and pleural cavities because of the evolution of a true 

 diaphragm separating the perivisceral coelom from the thoracic 

 cavities (fig. 18a). This structure is unique, for although similar 

 muscular organs are present in other animals they are not homo- 

 logous with the mammalian diaphragm. It is dome-shaped 

 anteriorly and is attached to the lumbar vertebrae and posterior 



